[pjc50]

Doing this as a person is fine. Storing and processing that information on a computer is covered by the DPA.

https://ico.org.uk/for-organisations/guide-to-data-protectio...

Refers to Schedule 2: http://www.legislation.gov.uk/ukpga/1998/29/schedule/2

Additionally, there are "Sensitive" personal data: http://www.legislation.gov.uk/ukpga/1998/29/section/2

So if someone were to ask your computer "Do you know any trade unionists?" and it were to reply "I know this person. His name is batou.", and you weren't covered by the Schedule 3 exceptions, that would be an offence. This is an attempt at preventing employment blacklists.